1. Field of the Invention
The present invention relates to a signal transmission device, and in particular to a signal transmission device that uses optical signals.
2. Description of the Related Art
In accompaniment with the increase in the resolution of liquid crystal panels and plasma displays, there is a demand to transmit, as digital signals, large-capacity image signals from a host. For example, in the Digital Visual Interface standard (DVI: a digital image transmission standard), which was established by the Digital Display Working Group (DDWG) that is an industry group, high-speed signals of 1.65 Gbps per 1 bit are transmitted using a differential signaling specification called transition minimized differential signaling (TMDS). A shield-attached metal cable widely used as a display cable is used as the transmission medium.
FIG. 11 shows an example where a host computer 86 and a monitor 88 are connected by a DVI cable 71 and image signals from the host computer 86 are displayed on the monitor 88. FIG. 12 shows a configural diagram of the DVI cable 71. TMDS signals in the DVI cable 71 are 4-bit differential signal pairs, and R, G and B of image signals and pixel clocks are allocated. The DVI cable 71 has a maximum transmission speed of 1.65 Gbps depending on the resolution of the image. Display data channel (DDC: display information) signals comprise (DDC Clock) and (DDC Data), and the display information (DDC) is exchanged between the host computer 86 and the monitor 88. A 5V Power signal and a hot plug detect signal (HPD: corresponding signal of the monitor) are level signals that give notification of the connection status of the host computer 86 and the monitor 88. When the host computer 86 starts up, the 5V Power signal becomes a high level, and when the 5V Power signal is inputted to the monitor 88 in a state where the host computer 86 and the monitor 88 are connected, the HPD signal becomes a high level. In addition to these, shield (GND) lines are plurally disposed in the metal DVI cable 71.
However, the length of the DVI cable 71 cannot be stretched more than 10 m because the signals that the DVI cable 71 handles are high speed. For this reason, cables (fiber cables, etc.) that convert high-speed signals to optical signals to realize long-distance transmission have also been proposed, but problems that must be resolved remain in terms of power consumption and safety in handling a laser light source for the optical signals.
For example, laser light is used in these optical transmission devices for emission light from the light source, but when there is unintentional cutting of the fiber cable or removal of the optical connector during operation, there is the potential for the laser light to leak to the outside and cause damage to human eyes. For this reason, technology that incorporates a safety circuit to stop laser light emission when the optical cable is unplugged has been proposed (e.g., see Japanese Patent Application Laid-pen Publication (JP-A) No. 2001-185783).
However, with respect to cables in which optical fiber and electrical wiring are mixed together, problems remain in that, when cutting of the light is determined only by the connection status of the electrical wiring, the cutting cannot be grasped when only the fiber cable has been cut, and the light emission cannot be appropriately stopped. Thus, it is actually preferable to monitor the reception status of the optical signal and stop light emission when normal optical signal is not being received. As a method of determining the status of transmission from the optical reception unit, technology has been proposed in the field of bi-directional optical communication that monitors the reception status of the optical signal and detects abnormalities (e.g., see Japanese Patent No. 2838454). By reflecting this technology in the aforementioned safety circuit that stops the laser light emission, it is possible to determine the connection status of the electrical wiring and the fiber cable and stop the laser light emission.
Also, as technology that achieves power saving in relation to optical extension cables, technology has been proposed that turns the power ON in accordance with the startup of the host in a state where the host and the monitor are normally connected (e.g., see JP-A No. 2003-209920). This technology proposes shutting down the power supply when the host computer and the monitor are not connected, to thereby reduce unnecessary power consumption. Thus, as long as the host computer and the monitor are normally connected, the optical transmission unit also starts up together with the startup of the host computer resulting from the power being turned ON, and the power supply of the optical transmission unit is also cut off due to the power of the host computer being cut off.
However, with respect to optical transmission devices, the function of detecting abnormalities in the optical signals and rapidly handling them and the function of achieving power saving are conflicting functions. Namely, the function of achieving power saving is technology that contradicts the safety circuit monitoring the actual optical signals. For example, sometimes the optical signal does not reach the reception circuit and, as a result, sometimes it is perceived that trouble has arisen during the time of the system startup from when the host computer is turned ON until the image signal is outputted when the initial setting of the system is being conducted (the so-called time lag), or in cases where the image signal stops when the resolution of the image is switched, or even in cases where the host computer temporarily stops the image signal within the range of normal operation.